The present invention relates to the compressed-air breathing apparatus in particular for underwater diving comprising compressed air supply means communicating with a compressed-air expansion chamber through a valve the opening and closing of which are controlled by means responsive to the inhaling effort of the user, and a mouthpiece including a conduit which puts the expansion chamber in communication with the mouth of the user.
Modern compressed-air breathing apparatus are generally of the type having two expansion stages. The compressed air of the cylinders carried by the diver is expanded first of all from a high pressure (about 200 bars) to a pressure termed an intermediate pressure (about 9 bars) by a pressure reducing valve fixed to said cylinders. This air at intermediate pressure is supplied through a flexible pipe to a second stage which is connected to the mouth of the diver by the mouthpiece.
In this second stage, the air is expanded from the intermediate pressure to the ambient pressure by a mechanism actuated by the depression created by an air demand on the part of the diver. The air flow delivered by the second stage as a function of the depression applied thereto by the air demand on the part of the diver characterises the performance of a breathing apparatus.
A second-stage pressure-reducing valve will be considered generally to be all the more pleasant for the user and better in performance as the depression required for obtaining a given rate of flow will be low. However, this type of equipment must be capable of satisfying the requirements of different types of users. When the users are professional divers, in particular when it concerns divers working under water and making physical efforts, it is desirable to arrange that these divers make a minimum inhaling effort to obtain the supply of breathing air. For this purpose, the opening of the valve must occur under a minimum depression. On the other hand, when this equipment is used in underwater diving schools, it is on the contrary necessary to arrange that the valve open under an increased depression, i.e. a large inhaling effort, so as to avoid an excessively strong assistance to said users. Further, depending on the depth at which the equipment is used, the air flow varies, namely it increases with the depth at which the diver is positioned.
Further, notwithstanding close manufacturing tolerances, a small difference in the mechanical characteristics of the elements controlling the valve of the breathing apparatus may result in a large difference in the inhaling effort on the part of the diver everything, else being equal.